Antennas: vertical and horizontal polarization; concept of gain; common portable and mobile antennas; relationships between antenna length and frequency

T9A01

What is a beam antenna?

A.

An antenna built from aluminum I-beams

B.

An omnidirectional antenna invented by Clarence Beam

C.

An antenna that concentrates signals in onedirection

D.

An antenna that reverses the phase of received signals

T9A01

The term "beam antenna" is just another name for a directional
antenna; it's an antenna
that concentrates signals in one direction. You can think of it that it
"beams" the signals in a certain direction.

This may be in common usage but is not good practice to identify an antenna. A more accurate terminology is high gain, directional, or electrically large antenna.

Remember that with an electromagnetic (RF) signal, it is the electrical
field that emanates outwards parallel to the originating antenna, which means
with a vertical antenna it is the electric (not the magnetic!) field that is
perpendicular to the Earth.

Using the Right Hand Rule of electromagnetic fields would tell you that if the fingers wrap in the direction of the magnetic (B) field, the thumb will point in the direction of the electric (E) field.

Which of the following describes a simple dipole mounted so the conductor is parallel to the Earth's surface?

A.

A ground wave antenna

B.

A horizontally polarized antenna

C.

A rhombic antenna

D.

A vertically polarized antenna

T9A03

The orientation of the conductor of an antenna relative to the earth's surface
determines its "polarization". If the polarization of the sending station's
antenna does not match the polarization of the receiving station's antenna
significant loss in signal can be the result.

If the antenna is vertical (perpendicular to the ground) as most antennas are
thought to be then it is "vertically polarized" and if it is horizontal
(parallel to the ground) then it is "horizontally polarized".

What is a disadvantage of the "rubber duck" antenna supplied with most handheld radio transceivers?

A.

It doesnot transmit or receive as effectively as a full-sized antenna

B.

It transmits a circularly polarized signal

C.

If the rubber end cap is lost it will unravel very quickly

D.

All of these choices are correct

T9A04

Smaller antennas use electrical components to maintain resonance on the target
frequencies, but having less surface area they don't absorb (or emit) as much
power. Therefore they do not transmit or receive as effectively as a regular
full-sized antenna.

As a general rule of thumb, the shorter the antenna on a given band the worse
the performance will be and the longer the better. Of course, other factors
such as the resonance of the antenna on the frequencies used can also affect
this!

The only reasons to use rubber duck type antennas are that they take up less space and are usually more durable than longer antennas. This makes it easier to keep an HT on your belt compared to a possibly much longer antenna. These sorts of tradeoffs are more typically worthwhile for commercial users such as security guards who will be near a repeater or other HT users almost all the time, so they aren't nearly as concerned as much about gain as most amateur operators.

These are all examples of "beam antennas", also called Directional
Antennas. Yagi are the most
common type in ham radio and you've probably seen TV antennas that are yagi
antennas; they have long elements in the back and short ones in the front and
make a sort of V shape with their outline. Dish antennas, such as those
commonly used by satellite TV systems, are another type are much more
obviously directional.

What is a good reason not to use a "rubber duck" antenna inside your car?

A.

Signals can be significantlyweaker than when it is outside of the vehicle

B.

It might cause your radio to overheat

C.

The SWR might decrease, decreasing the signal strength

D.

All of these choices are correct

T9A07

A rubber duck antenna is a sub-performant antenna to start out with, but when
you're inside your car you are surrounded by a metal shield that impedes the
RF energy to and from your radio, which means that the signal will often be
significantly weaker than if you were outside of your vehicle. In addition
some rubber duck antennas are too long to be held vertically which changes the
polarization of the signal and causes additional loss.

One easy solution to this is to get a cheap magnetic mount antenna that can be
placed on top of your car and then connected to your handheld radio. These can
be found for under $20 on ebay.

What is the approximate length, in inches, of a 6 meter 1/2-wavelength wire dipole antenna?

A.

6

B.

50

C.

112

D.

236

T9A09

There are many factors that will affect the amount of length needed for the
1/2 wave dipole antenna, such as the physical characteristics of the wire or
nearby conductive sources. But the easiest way to solve this problem is to
remember that a meter is a little longer than a yard, or approximately 39
inches. To calculate this, half of the 6 meter wavelength would be 3 meters.
To convert that to inches, multiply by 39 inches per meter:

With 12 inches in a foot, you get \[9.36\text{ ft}\times12\text{ in} = 112.32\text{ in}\]

Quarter wavelength dipole is the same, but divide 234 by the frequency in
megahertz. This is easier to remember (since 234's digits are sequential), so
just remember that one and convert up when needed!

The additional power that is lost in the antenna when transmitting on a higher frequency

C.

The increase in signalstrength in a specifieddirection when compared to a reference antenna

D.

The increase in impedance on receive or transmit compared to a reference antenna

T9A11

Think of gain as a focusing quality of an antenna, like the reflector on a
flash light.

By the geometry of the antenna we can change how the antenna emits radio
waves, or RF energy. We can focus it like a spot light by using a yagi
antenna, or we can let it flood out more evenly like a room shop light, by
using a dipole antenna.

The higher the gain, the more focused the beam of RF energy, which results in
an increased signal strength in a particular direction.

The 5/8 wavelength antenna offers a 10-times power gain over a 1/4 wavelength design

T9A12

Lots of interesting distractors, but the simple fact is that a 5/8 wave radiates at a lower angle than a 1/4 wave antenna, which is usually a good thing. So just remember 5/8 is larger than 1/4 so 5/8 is better. Makes it easy to remember the correct answer

What about the distractors? Well, a 1/4 wave antenna might be better for reaching repeaters, but a 5/8 wave antenna is not.

No matter what angle your signal is radiated, you're likely to encounter reflected signals, so a 5/8 wave antenna doesn't eliminate those. In fact, if it sends the signals into nearby mountains, you're more likely to encounter reflected signals with the 5/8 wavelength antenna!

To get "power gain" of 10-times the power, compared to a 1/4 wave antenna, you'd have to focus the signal so that it appears 10 times as strong, at the lower angles of radiation. While a 5/8 wave antenna will, indeed, exhibit an increase in signal strength at the lower angles, that increase is in the order of 3dB, (2-times the power) nowhere near the 10dB of a 10-times increase.

Mounting on the roof of the car vs any other location should not significantly impact the SWR of a properly designed antenna, and anywhere you install an antenna vertically will give you a vertically polarized signal.

In addition, roof mounting is often more difficult than other locations, since it requires finding a mount (such as a magnetic mount or luggage rack mount) that will position the antenna correctly.

The two most common methods of installing an antenna in the center of the roof are magnetic mount (which can damage paint, but is not permanent) and drilling a hole. Obviously the latter is far from the easiest, but it is one of the most reliable.

Being centered above the car tends to give the antenna the best and most uniform radiating pattern, since there are no odd metal pieces in the way.

Which of the following terms describes a type of loading when referring to an antenna?

A.

Inserting an inductor in the radiating portion of the antenna to make it electricallylonger

B.

Inserting a resistor in the radiating portion of the antenna to make it resonant

C.

Installing a spring at the base of the antenna to absorb the effects of collisions with other objects

D.

Making the antenna heavier so it will resist wind effects when in motion

T9A14

Inductors in series make an antenna appear electrically longer. So you'd insert an inductor into the radiating portion of the antenna to make it appear electrically longer.

Adding a resistor will reduce current flow, but it wouldn't affect the resonant frequency.

The spring at the base of an antenna would absorb the effects of collisions with other objects, but absorbing collisions has nothing to do with loading. It might make the antenna slightly longer, especially at higher frequencies, but the distractor doesn't say anything about that.

Resisting wind effects has to do with what can be known in civil engineering as "wind loading" - but that's not what they're referring to when they talk about loading.